Bimetallic elements having heat-expansion characteristic

ABSTRACT

A THERMOSENSITIVE ELEMENT COMPRISING A FIRST LAYER AND A SECOND LAYER OF A NON-CUBIC METAL HAVING A HIGH DEGREE OF CRYSTALLOGRAPHIC ORIENTATION IN WHICH THE DIRECTIONS OF CRYSTALLOGRAPHIC ORIENTATION ARE AT RIGHT ANGLES TO EACH OTHER.

United States Patent ABSTRACT OF THE DISCLOSURE A thermosensitiveelement comprising a first layer and a second layer of a non-cubic metalhaving a high degree of crystallographic orientation in which thedirections of crystallographic orientation are at right angles to eachother.

The instant invention is directed to devices constructed of temperaturesensitive materials and, more particularly to thermosensitive elementscomprising two layers of material each having the same chemicalcomposition. Such elements can be conveniently described as monometallicelements.

Thermosensitive elements and devices now available are generally of thebimetal type. A bimetallic thermosensitive device is a laminatecomprising two or more layers of material, usually metals, havingdifferent linear thermal expansion coefficients. The laminate tends tochange curvature with changes in temperature. Such thermosensitiveelements are employed in thermometers, and temperature control equipmentwhich are operative up to temperatures of approximately 1000 C. However,the bimetal thermosensiti-ve devices presently available are notsatisfactory for use at temperatures in excess of about 800 F.,particularly for prolonged periods of time.

One of the most serious problems encountered in the use of ordinarybimetallic elements at high temperatures is caused by diffusion acrossthe interface between the layers. If a sufficiently long period of timewere allowed, diffusion would produce a homogeneous strip having nothermal sensitivity. In shorter periods of time, particularly atelevated temperatures, diffusion does cause effective changes in thethermal activity of the bimetal element either by reducing thedifference between the coefficients of linear expansion or by thegradual formation of an interfacial layer having a composition andthermal characteristics different from one or both of the originalstrips making up the bimetal.

It can therefore be appreciated that it would be very advantageous iftemperature sensitive elements could be produced comprising two layersof strips of metal having the same chemical composition but havingdifferent thermal properties. As used herein, including the appendedclaims, the term metal is intended to include metals and alloys ofmetals.

It is therefore an object of the present invention to provide a twolayer temperature sensitive element which is not subject to diffusionalintercontamination.

It is an object of this invention to provide metallic temperaturesensitive devices suitable for prolonged use at very high temperatures.

It is still a further object of the invention to provide a temperaturesensitive element which is not subject to loss of sensitivity afterprolonged use at high temperatures.

These and other related objects are achieved by providing a thermostatmaterial, i.e., a temperature sensitive element, comprising two layersof metal having the same chemical composition and different thermalexpansion properties bonded together to form a laminate of homogeneouscomposition. Such materials, i.e., those having the 3,591,353 PatentedJuly 6, 1971 same or very similar chemical composition and differingthermal linear expansion properties can be conveniently referred to ascrystallographically textured materials. The term crystallographicallytextured refers to materials having a substantial degree of preferredcrystal orientation.

A metal sheet is simply a composite of an extremely large number ofindividual grains, each of which may be considered to be a crystal. Thethermal expansion of a sheet cannot differ more with direction than themaximum difference in thermal expansion between two directions in acrystal of that material. For example, magnesium at approximately roomtemperature has thermal expansions of 26.5 and 25.1 10- per degreecentigrade in the two crystal directions. This difference is too smallto produce a workable deflection in a thermally active element. Anappreciable difference, (on the order of at least about 10%) in thermalexpansions between different crystallographic directions is necessaryfor most practical applications.

A cubic crystal expands equally in all directions and is therefore notapplicable. Such common metals as iron, chromium, nickel, copper, gold,silver, tungsten, molybdenum, aluminum, platinum, tantalum and thoriumare cubic metals and thus fail to quality in the present invention.

A further requirement is that there not be any change in crystalstructure between room temperature and the service temperature. Cobaltand uranium undergo allotropic transformations when subjected totemperatures not substantially above 750 F. and 1200 F., respectively.Preferred crystallographic orientation would, to a large extent, beeliminated with the allotropic change and therefore these metals couldnot be used in applications where the maximum temperature exceeds theallotropic transformation temperature. Notably among the metals whichmight be used for bimetallic-type elements based on preferredcrystallographic orientation and an ability to function after beingsubjected to a temperature of 1400 F. or below are beryllium, osmium,rhenium, ruthenium, titanium, yttrium, and zirconium.

Further exclusionary factors are high temperature strength, oxidationresistance, workability, weldability, or rate of loss preferredorientation as a function of time and temperature.

It is also necessary that the melting point be well above the requiredservice temperature, For example, in a 1400 F. application, magnesiumwould be excluded from interest because of its relatively low meltingpoint (1202 F.). Some of the other metals which would be similarlyexcluded are antimony, bismuth, cadmium, indium, tellurium, thallium,tin and zinc.

In preparing temperature sensitive devices of the type described above,an upper and lower layer of chemically identical or similar metal havingdifferent directions of crystal orientation, with respect to theirlength, are bonded together to form a laminate in which the directionsof orientation are not parrallel. Preferably the directions oforientation should be at right angles to each other. The laminate isthen incorporated into a temperature sensitive device in the usualmanner.

In general metals and alloys which are crystallographically anisotropic,e.g., hexagonal metals such as titanium and zirconium, are characterizedby measurable differences in thermal expansion along eachcrystallographic axis. Frequently the anisotropy, a characteristicgenerally considered to be detrimental, can be increased by suitabletreatment of the metal, e.g., by suitably annealing the metal or bysubjecting the metal to additional rolling. By way of example, a sheetof non-cubic alloy can be rolled to provide a sufficient degree ofpreferred crystal orientation, i.e., in the direction of rolling, andthen pieces of corresponding size are cut from the rolled sheet, one inthe longitudinal direction (the direction of rolling) and one in thetransverse direction (at right angles to the direction of rolling). Thetwo pieces are then laminated together, with their directions oforientation at right angles.

More particularly, it has been discovered that titanium alloys show asubstantial difference between the coefficient of linear thermalexpansion measured in the longitudinal direction and the coefiicientmeasured in the transverse direction.

Anisotropic thermal expansion in titanium alloys, e.g., an alloycontaining about 8% aluminum, about 1% molybdenum, and about 1%vanadium; has been investigated and found to be of a magnitude suitablefor use as a temperature sensitive material.

Table I, below, shows the results of dilatometric measurement of thethermal expansion in both the longitudinal and transverse direction onthe above-mentioned aluminum-molybdenum-vanadium containing titaniumalloy.

TAB LE I Mean expansion coefficient between room-temperature and thetemperature shown in column 1 per degree 0.)

Temperature, 0. Longitudinal Transverse TABLE II Mean expansioncoefficient between room temperature and the temperature shown in column1 (10- per degree 0.)

Temperature) 0. Longitudinal Transverse Table III, below, shows theeifect of solution annealing for one hour at 1850 F.

TABLE III Mean expansion coetficient between room temperature and thetemperature shown in column 1 (10- per degree 0.)

Transverse Temperature, 0. Longitudinal The temperature sensitiveelements herein described can be easily prepared by cutting, at rightangles to each other, thin fiat strips of metal from a sheet of suitablytextured, i.e., crystallographically oriented metal. The thin strips arethen bonded together, for example by riveting, welding, or diffusionbonding, to form a flat composite in which one of the strips has thedirection of crystallographic orientation running at right angles todirection of orientation in the first strip, i.e., across the width ofthe strip. Due to the difference in thermal expansion between the twolayers of the composite, the flat composite element bends into a curvewhen heated. Such a composite can be employed as the operative member ofa thermostat, one end of the composite being fixed to a suitable supportand the motion of the unfixed end being utilized to open and close anelectrical circuit.

The temperature sensitive elements herein described can also be employedin thermometers, e.g., in the form of a helix which winds and unwinds inresponse to changes in temperature, the movement being indicated by apointer which travels over a calibrated dial.

What is claimed is:

1. A thermosensitive element comprising a bonded composite of first andsecond layers of crystallographic textured non-cubic metals having ahigh degree of crystallographic orientation in which the directions ofcrystallographic orientation are at right angles to each other toproduce a workable deflection in the element in response to temperaturechange.

2. A temperature sensitive element comprising a first layer of acrystallographically textured non-cubic metal and a second layer of thesame metal, said first layer bonded to said second layer in differentcrystallographic directions to produce a workable deflection in theelement in response to temperature change.

3. A temperature sensitive element comprising a first layer of atextured crystallographic titanium alloy and a second layer of the sametextured titanium alloy, said first layer bonded to said second layer indiiferent crystallographic directions to produce a workable deflectionin the element in response to temperature change.

4. The temperature sensitive element of claim 3 wherein the titaniumalloy contains about 8 weight percent aluminum, about 1 weight percentmolybdenum, and about 1 weight percent vanadium.

References Cited UNITED STATES PATENTS 1,985,181 12/1934 Matthews29-195.S 2,234,748 3/ 1941 Dean et al. 29--195.5UX 2,940,163 6/1960Davies 29198X 3,156,978 11/1964 Hanink et al 29 -198X FOREIGN PATENTS209,931 8/1957 Australia -1755 ALLEN B. CURTIS, Primary Examiner US. Cl.X.R.

